1. Field of the Invention
The invention relates to a method and apparatus for calibrating NMR imaging and imaging spectroscopy devices. The apparatus establishes a precise three-dimensional grid. One embodiment allows three-dimensional, measurable fluid flow through the grid. A second embodiment is easily reassembled by the user.
2. Prior Art
The necessity of calibrating NMR imaging devices has long been recognized. Most such calibration is achieved by imaging a phantom. A phantom is a device of known characteristics which can be placed in the NMR imaging device and which produces a predictable image.
Previous phantoms have allowed for two-dimensional calibration, i.e. calibration in a single plane. However, in practice, the NMR imaging devices are used to image objects having three dimensions, such as the organs of the human body. Unlike CT imaging, which is only capable of collecting multiple two-dimensional slices, NMR is a true three-dimensional technique. For instance, double angulation can be done with NMR machines, or data can be collected from a volume and later reconstructed into slices at arbitrary angles.
Some three-dimensional or pseudo three-dimensional phantoms have been developed. For example, U.S. Pat. No. 4,625,168, issued to Meyer et al, shows a cylinder with grooves forming a grid. The grooves contain some material different from the cylinder and therefore produce a contrasting NMR image.
Nevertheless certain problems remain. One problem is the difficulty of machining solid phantoms. Known plexiglass phantoms, for instance, prove very expensive. Moreover, such phantoms are usually composed of stacks of two-dimensional structures rather than providing a true three-dimensional metric. They do not easily provide calibration for double angulation, because they do not provide similar grids in all three dimensions.
Another problem that remains is calibrating the spatial distributions of three-dimensional fluid flow and rate of fluid flow. NMR machines are particularly useful in medical applications, because they are able to distinguish many different fluids, and track the movement of those fluids through three-dimensional space. A number of new and evolving medical applications require measuring T.sub.1 and T.sub.2 relaxation times in moving fluids in three-dimensional space. Other applications in magnetic resonance spectroscopy (MRS) require taking spectra of static and moving fluids from various places in three-dimensional space, e.g. the confines of an organ of the body.
Yet another problem is that most phantoms are fixed. They do not allow the user to reshape them into useful alternative three-dimensional shapes.
One example of medical applications requiring calibration of three dimensional fluid flow is the study of the flow and perfusion of blood through the brain or heart.
An example of an application requiring accurate three-dimensional localization of a sample volume is measuring phosphorous 31 spectra at different locations in an organ. The relative concentration of phosphorous metabolites: ATP, phosphocreatine (PCR) and inorganic phosphate (Pi) define the energy state of cells. In addition, intracellular pH can be determined by the spatial chemical shift (altered position in the spectrum of the inorganic phosphate peak).
Another example of an application requiring both accurate localization and known flow is determining the effects of blood perfusion on spectroscopic saturation transfer measurements.
For these and other applications, being certain of the actual small localized volume within the body that was sampled is essential for the interpretation of results.
In the prior art, a rack of small bottles was commonly used to calibrate T.sub.1, T.sub.2 characteristics. The bottles typically contained solutions of paramagnetic ions, such as Cu2.sup.+, Cr3.sup.3, Fe3.sup.+, Mn2.sup.+, Gd.sup.+, in concentrations ranging from 10.sup.-5 to 10.sup.-1 mol/liter. But this gives only a two-dimensional distribution. An example of phantoms made from bottles, containing U-shaped tubes for fluid flow, is found in P. R. Moran et al, "Verification of Internal Flow and Motion", Radiology 1985; 154: 433-441. This article also discusses the importance of fluid flow to NMR medical applications.
The Meyer patent also uses fluids having different values for T.sub.1 and T.sub.2 to calibrate sensitivity of an NMR device to these different values. However, the fluids in the Meyer patent are static.
Phantoms consisting of two or three coaxial or concentric chambers have been constructed giving three-dimensional or pseudo three-dimensional distributions, but these are difficult to build, provide limited dimensional information, and will not support flow studies.
Thus, at the present time calibration for many applications is impossible.